1. Field of the Invention
This invention relates to a hermetic scroll fluid discharge apparatus.
2. Description of the Prior Art
Scrool fluid discharge apparatus are known as from U.S. Pat. Nos. 3,884,599 and 3,924,977. The principle of operation of a scroll compressor, which is one example of the scroll fluid discharge apparatus, will be described first of all by referring to FIGS. 1a-1d.
In FIGS. 1a-1d,1a is a fixed scroll wrap, 1b the center of a fixed scroll member, 2a an orbiting scroll wrap, 2b the center of an orbiting scroll member and 3 a discharge port. 4 designates fluid pockets formed by the fixed scroll member and the orbiting scroll member maintained in intimate contact with each other in superposed relation. In FIG. 1, the fixed and orbiting scroll wraps 1a and 2a are perpendicular to end plates of the scroll members and mirror-like surfaces at the forward ends of the wraps 1a and 2a move in sliding movement on the end plates. The end plates of the fixed and orbiting scroll members are not shown in FIGS. 1a-1d.
The orbiting scroll wrap 2a moves in such a manner that the center 2b of the orbiting scroll member orbits around the center 1b of the fixed scroll member with a radius of orbiting of .epsilon., so that the fluid pockets 4 have their volume gradually reduced as shown in FIGS. 1a-1d.
The volume of the fluid pockets 4 is maximized when the fluid pockets 4 are in the condition shown in FIG. 1d, and the fluid that has its pressure maximized is led through the discharge port 3 to outside.
Owing to the pressure of the fluid in the fluid pockets 4, a force tending to urge the orbiting scroll member away from the fixed scroll member (hereinafter referred to as an axial biasing force) acts on the orbiting scroll member. Thus, it is necessary that the scroll compressor be provided with axial sealing means for forcing the orbiting scroll member against the fixed scroll member against the axial biasing force. Axial sealing means of the prior art will now be described.
FIG. 2 is a sectional view of a scroll compressor of the prior art. The scroll compressor shown in FIG. 2 is disclosed in Japanese Patent Application Laid-open No. 119412/78 and the corresponding application filed in the United States of America is identified as Ser. No. 887,252, now abandoned.
In FIG. 2, 1 is a fixed scroll member and 2 an orbiting scroll member. The two scroll members 1 and 2 are each provided with an end plate and have wraps 1a and 2a, respectively, arranged in vortical form on the respective end plates. 6 is a drive shaft for driving the orbiting scroll member 2, and 5 a main frame supporting the drive shaft 6 connected to motor 7. All the elements described hereinabove are contained in a hermetic container 8. 9 is a suction port for introducing fluid. 10 is an Oldham's ring for preventing the orbiting scroll member 2 from rotating on its own axis about its center (indicated at 2b in FIG. 1b). As described hereinabove, the orbiting scroll member 2 moves in such a manner that the center thereof orbits around the center of the fixed scroll member 1 and does not move about its own center axis. 11 is a lubricant in the hermetic container 8.
In the scroll compressor of the prior art constructed as aforesaid, means is provided for providing axial seal to the fixed and orbiting scroll members 1 and 2 by keeping the pressure in the hermetic container 8 at a predetermined level so that the pressure forces the orbiting scroll member 2 against the fixed scroll member 1 to attain the end. Such means comprises communication ports 2d for introducing a portion of the fluid under pressure in the fluid pockets 4 therethrough into the hermetic container 8 to increase the pressure therein. The communication ports 2d in FIG. 2 and formed in an end plate 2c of the orbiting scroll member 2. The pressure in the fluid pockets 4 in the compression stroke is led through the communication ports 2d to the hermetic container 8 to increase the pressure therein. FIGS. 3a and 3b show, on an enlarged scale, the essential portions of the compressor in the vicinity of the communication port 2d. The scroll compressor constructed as aforesaid has since been found to have the following disadvantages.
When the two scroll members 1, 2 are positioned as shown in FIG. 3a, the internal pressure in the hermetic container 8 is equal to the internal pressure in the fluid pocket 4a because communication is maintained through the communication port 2d between the fluid pocket 4a and the hermetic container 8. As evident from a review of FIGS. 1a-1b, the pocket 4a kept in communication with the hermetic container 8 through the communication port 2d is related in pressure to the fluid pocket 4b, 4c illustrated in FIG. 3a in such a manner that the pressure therein becomes higher in going toward the center of the scroll members so that the pressures in the sections or areas designated A and B in FIG. 2 are distinct from each other. In the section or area designated A, the fluid pocket 4c is on the center side and the compression stroke progresses in the order of 4b-4a-4c so that the internal pressure of the fluid pocket 4c is higher than the internal pressure of the fluid pocket 4a and the internal pressure of the fluid 4b is lower than that. In the section or area designated B in FIG. 2, the situation is in reverse for the fluid pocket 4b is on the center side and the compression stroke progresses in the order of 4c-4a-4b so that the internal pressure of the fluid pocket 4b is higher than the internal pressure of the fluid pocket 4a and the internal pressure of the fluid pocket 4c is lower than that.
When the compression stroke progresses, corresponding to section or area B in FIG. 2, and the two scroll members 1 and 2 are in the position shown in FIG. 3b, the communication port 2d should be completely closed. However, since the communication port 2d is formed in the end plate 2c of the orbiting scroll member 2, it is the mirror-like surface of the forward end of the fixed scroll wrap moving in sliding movement on the end plate 2c that closes the communication port 2d. The portion of the surface of the end plate 2c surrounding the communication port 2d is in contact with the mirror-like surface of the fixed scroll wrap in a zone corresponding to the thickness of the fixed scroll wrap 1a minus the diameter of the communication port 2d, so that, taking into account the pressure relationship between the fluid pockets 4a, 4b, 4c, namely, that the internal pressure of the fluid pocket 4b is less than the internal pressure of the pocket 4a, the fluid tends to leak, as indicated by an arrow A, in FIG. 3b in a direction toward the fluid pocket 4b from the hermetic container 8. As shown in FIG. 3c, corresponding to section or area B in FIG. 2, since the internal pressure of the fluid pocket 4b is higher than that in the fluid pocket 4a, in contrast to the pressure relationship in FIG. 3b, a fluid leak represented by the arrow A.sub.2 is directed from a fluid pocket 4b of higher pressure to the hermetic container 8 to thereby increase the internal pressure therein. If the fluid leaks in these two situations are equal to each other in amount, they would cancel each other out and the pressure level in the hermetic container would be kept constant. However, this in not the case because the pressure differentials that influence the fluid leaks differ from each other. An increase in the internal pressure of the hermetic container 8 results in an increase in the axial biasing force urging the orbiting scroll member 2 against the fixed scroll member 1. A rise in this axial biasing force above an optimum biasing force level causes an increase in mechanical loss that may reduce heat insulating efficiency and a breakage of an oil film that may cause seizure or galling in the sliding surfaces. As shown in FIG. 8, generally, a compression stroke 1-2-3-4-5 is expressed as PV.sub.K =C, where C is constant, P is the pressure, V is volume, and K is adiabatic indes of the fluid (K&gt;1). The numbers 1-5 in FIG. 8 indicate the various stages of the fluid pockets of the compressor. Assuming that the communication port is formed in the position designated by the number 3 in FIG. 8, and the internal pressure of the hermetic container 8 is kept at a pressure P.sub.n, a pressure differential that influences the fluid leak is such that, as shown by the arrow A.sub.1 in FIG. 3b, a fluid leak occurs from the hermetic container 8 to the fluid pocket designated by the number 2 in FIG. 8 of a lower pressure so that a pressure differential in this situation would be .DELTA.P.sub.1.
As shown by the arrow A.sub.2 in FIG. 3c, a fluid leak occurs at the fluid pocket represented by the numeral 4 in FIG. 8 of a higher pressure level to the hermetic container 8 resulting in a pressure differential of .DELTA.P.sub.2. Thus, the pressure differentials have a relationship of .DELTA.P.sub.1 &lt;.DELTA.P.sub.2 at all times. This means that, assuming other conditions affecting fluid leaks such as, for example, leak area, lubrication, etc. are equal, a fluid leak acts at all times in a manner to raise the pressure in the hermetic container 8.